In 3GPP networks, most of the channels (e.g, DPSCH, DPCCH, PBCH etc) are used to carry information as a sequence of bits in which they have some higher layer channel connected to them. However, a reference signal is a special signal that exists only in the Physical (PHY) layer. The purpose of this reference signal is to deliver a reference point for the downlink power.
When the UE tries to determine the downlink (DL) power (i.e, the power of the signal from an eNode B), the UE measures the power of this reference signal and uses this measured power as the downlink cell power. These reference signals are carried by multiple specific resource elements in determined slots, and the location of the resource elements are specifically determined by antenna configuration. FIG. 1 illustrates example time slots in which the reference signal is carried, and the resource elements marked in gray are the ones reserved for the reference signal, but are not carrying the reference signal for that specific antenna.
There are two different types of reference signals: the Cell Specific Reference Signal (CRS) and the UE Specific Reference Signal. The CRS is being transmitted at every subframe and it spans across the entire operating bandwidth. Generally, the CRS is transmitted by Antenna port 0,1,2,3. The location of the Cell specific Reference Signal varies according to Physical Cell ID. The downlink cell specific reference signal can be used for (i) cell search and initial acquisition; (ii) downlink channel quality measurements; and (iii) downlink channel estimation for coherent demodulation/detection at the UE.
For decoding any downlink data, the first step is to detect/decode the CRS transmitted from the eNodeB. If the power of the CRS is the same as all other power channels, it would be extremely difficult to detect the CRS. Thus, some implementations make the power of the CRS large as compared to other channels as shown in the black bar in FIG. 2 in which an offset (P_A) between the CRS and other channels exist.
However, because the reference channels occur only in specific symbols instead of every symbol, a complication exists in which there are some symbols with the reference signal in it, and there are some other symbols without reference signal in it. When this situation occurs, if the power of each symbol is measured, some symbols (i.e., symbols with the CRS) have higher power than the other symbols (i.e., symbols without CRS), which complicates the implementation of the receiver equalizer.
In one attempt to solve the problem of the power difference between two groups of symbols, lesser power (P_B) can be applied to the non-reference signal channels at the symbol carrying reference signal. However, in this solution, there is a complicated peak-and-valley type of power terrain rather than the flat plain terrain in downlink power allocation. This complicated peak- and valley type of power terrain requires a more complicated receiver implementation.
One important usage of the CRS by UEs is to perform cell search and initial camping. In other words, the received signal strength of the CRS may impact UE distribution in certain cells. Current solution has the same CRS power adjustment for all cells. In scenarios with unevenly distributed load (i.e., some cells are over heavily loaded while other neighboring cells are lightly loaded) the uniformly boosted CRS power does not help with load balancing.